Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
In an optical network, laser light is modulated to represent digital data. Intensity-modulated Phase-shift Keying (IM-mPSK) modulation is one type of modulation that involves shifting the phase of the output waveform to one of a fixed number of states. Three common versions are intensity-modulated binary, or BPSK, quadrature, or QPSK, and 8Ø-PSK, corresponding to two, four and eight states, respectively. IM-mPSK is described in T. Miyano, M. Fukutoku, K. Hattori, and H. Ono, “Suppression of degradation induced by SPM/XPM+GVD in WDM transmission using bit-synchronous intensity modulated DPSK,” in Proceedings of OECC '00, 14D3, 2000; A. H. Gnauck et al., “2.5 Tb/s (64×42.7 Gb/s) transmission over 40×100 km NZDSF using RZ-DPSK format and all-Raman amplified spans,” in Proceedings of OFC'02, FC2-1, 2002; O. Vassilieva et al., “Numerical comparison of NRZ, CS-RZ and IM-DPSK formats in 43 Gbit/s WDM transmission,” in Proceedings of LEOS 2001, ThC2, pp. 673-674, 2001; and C. Wree et al., in Proceedings of ECOC 2002, paper 9.6.6, 2002.
Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that a signal can be transmitted in a WDM or other optical network without amplification is limited by absorption, scattering and other loss associated with the optical fiber. To transmit signals over long distances, optical networks typically include a number of discrete amplifiers spaced along each fiber route. The discrete amplifiers boost received signals to compensate for transmission losses in the fiber.
Optical signals may experience degradation due to fiber cuts, signal attenuation, fiber deterioration, excess noise, temporal deviation of fiber dispersion, or other causes. Providing reliable communication in an optical network requires maintaining signal quality, often measured by bit error rate (BER). Thus, the BER of an optical signal, or an approximation thereof, is useful in monitoring the quality of the optical signal.